Abstract: in 1987, the 17D sub Technical Committee of the International Electrotechnical Commission (IEC) drafted a technical document entitled "supplement 1 to iec439 requirements on insulation coordination", which formally introduced the issue of insulation coordination into low-voltage switchgear and control equipment. As far as the actual situation in China is concerned, the insulation coordination of equipment is still a big problem in high-voltage and low-voltage electrical products. It is only two years since the concept of insulation coordination has been formally introduced in low-voltage switchgear and control equipment. Therefore, it is an important problem to properly handle and solve the insulation coordination problem in products.
Key words: low voltage switchgear insulation coordination insulation materials
Insulation coordination is an important issue related to the safety of electrical equipment products, which has always been paid attention to from all aspects. Insulation coordination was first used in high-voltage electrical products. In 1987, sub Technical Committee 17D of the International Electrotechnical Commission (IEC) drafted a technical document entitled "supplement 1 to iec439 requirements on insulation coordination", which formally introduced the issue of insulation coordination into low-voltage switchgear and control equipment. As far as the actual situation in China is concerned, the insulation coordination of equipment is still a big problem in high and low voltage electrical products. Statistics show that the accidents caused by the insulation system account for 50% - 60% of China's electrical products. It is only two years since the concept of insulation coordination has been formally introduced into low-voltage switchgear and control equipment. Therefore, it is an important problem to properly handle and solve the insulation coordination problem in products.
II Basic principle of insulation coordination
Insulation coordination means that the electrical insulation characteristics of the equipment are selected according to the service conditions of the equipment and the surrounding environment. Insulation coordination can be realized only when the design of the equipment is based on the action strength it bears in its expected life. The problem of insulation coordination not only comes from the outside of the equipment, but also from the equipment itself. It is a problem that involves various factors and must be comprehensively considered. Its main points are divided into three parts: first, the service conditions of the equipment; The second is the operating environment of the equipment, and the third is the selection of insulating materials.
（1） Service conditions of equipment
The service conditions of the equipment mainly refer to the voltage, electric field and frequency used by the equipment.
1. relationship between insulation coordination and voltage. In considering the relationship between insulation coordination and voltage, it is necessary to consider the voltage that may appear in the system, the voltage generated by the equipment, the required continuous voltage operation level, and the risk of personal safety and accidents.
1 Classification and waveform of voltage and overvoltage.
a) Continuous power frequency voltage with constant R, m and S
b) Temporary overvoltage, power frequency overvoltage with long duration
c) Transient overvoltage, an overvoltage with a duration of a few milliseconds or less, usually highly damped oscillatory or non oscillatory.
——Slow wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 20 μ s<Tp<5000 μ S, wave tail duration T2 ≤ 20ms.
——Fast wave front overvoltage: a transient overvoltage, usually unidirectional, with a peak time of 0.1 μ s<T1<20 μ s. Wave tail duration T2 ≤ 300 μ s。
——Steep wave front overvoltage: a transient overvoltage, which is usually unidirectional and reaches the peak value with TF ≤ 0.1 μ s. The total duration is <3ms, with superimposed oscillation, and the oscillation frequency is between 30khz<f<100mhz.
d) Combined (temporary, slow front wave, fast front wave, steep front wave) overvoltage.
According to the above overvoltage types, the standard voltage waveform can be described.
2 for the relationship between long-term AC or DC voltage and insulation coordination, the rated voltage, rated insulation voltage and actual working voltage shall be considered. During the normal and long-term operation of the system, the rated insulation voltage and the actual working voltage should be mainly considered, which should not only meet the requirements of the standard, but also pay attention to the actual situation of China's power grid. At present, the quality of China's power grid is not high. When designing products, the actual possible working voltage is more important for insulation coordination.
3. The relationship between transient overvoltage and insulation coordination is related to the controlled overvoltage conditions in the electrical system. In the system and equipment, there are many forms of overvoltage, so the influence of various overvoltage should be fully considered. In the low-voltage power system, overvoltage may be affected by various variable factors. Therefore, the overvoltage in the system is evaluated by statistical method, which reflects a concept of occurrence probability, and whether protection control is required can be determined by probabilistic method.
2.overvoltage category of equipment
According to the service conditions of the equipment and the required long-term continuous voltage operation level, the overvoltage category of the equipment directly supplied by the low-voltage power grid is divided into level IV. Overvoltage class IV equipment refers to the equipment used at the power supply end of the power distribution device, such as electricity meters and front-end current protection equipment. Overvoltage category III equipment is the task of installing in the power distribution device, and the use safety and applicability of the equipment must meet special requirements, such as the switchgear in the power distribution device. Overvoltage category II equipment refers to energy consuming equipment powered by power distribution devices, such as loads for household and similar purposes. The equipment of overvoltage category I is connected to the equipment that limits the transient overvoltage to a relatively low level, such as the electronic circuit with overvoltage protection. For the equipment not directly powered by the low-voltage power grid, the maximum voltage that may occur in the system equipment and the serious combination of various conditions must be considered.When the equipment needs to work in a higher overvoltage category, but the allowable overvoltage category of the equipment itself is not enough, it is necessary to take measures to reduce the overvoltage. The following methods can be used.
a) Overvoltage protection device
b) Transformer with isolated winding
c) Multi branch circuit distribution system with dispersive transfer of wave energy through voltage
d) Capacitor capable of absorbing surge overvoltage energy
e) Damping device capable of absorbing surge overvoltage energy
3. electric field and frequency
The electric field is divided into uniform electric field and non-uniform electric field. In the low-voltage switchgear, it is generally considered to be in the case of non-uniform electric field. As for the frequency, it is still under consideration. It is generally considered that the low frequency has little impact on the insulation coordination, but the high frequency still has an impact, especially on the insulating materials.
（2） Relationship between insulation coordination and environmental conditions
The macro environment where the equipment is located affects the insulation coordination. From the current actual application and standard requirements, the change of air pressure only takes into account the change of air pressure caused by altitude. The daily change of air pressure has been ignored, and the factors of temperature and humidity have also been ignored. However, if there are more precise requirements, these factors should also be considered. From the perspective of micro environment, the macro environment determines the micro environment, but the micro environment may be better or worse than the macro environment equipment. Different degrees of protection, heating, ventilation and dust of the shell may affect the micro environment. The micro environment is clearly specified in relevant standards, as shown in Table 1, which provides a basis for product design.
（3） Insulation coordination and insulation materials
The problem of insulating materials is quite complex. Unlike gas, it is an insulating medium that cannot be recovered once it is damaged. Even accidental overvoltage events may cause permanent damage. Insulating materials will encounter various situations in long-term use, such as discharge accidents. However, due to various factors accumulated for a long time, such as thermal stress, temperature, mechanical shock and other stresses, It will also accelerate its aging process. As for insulating materials, due to the diversity of varieties, there are many indexes to measure the characteristics of insulating materials, but they are not unified. This brings some difficulties to the selection and use of insulating materials, which is the reason why other characteristics of insulating materials, such as thermal stress, mechanical properties, partial discharge and other indicators, are not considered internationally at present. The influence of the above stress on insulating materials has begun to be discussed in IEC publications, which can provide some qualitative guidance for practical application, but quantitative guidance is not available at present. At present, among low-voltage electrical products, CTI value, which is divided into three groups and four categories, and PTI value, which is used as an index to quantitatively guide insulating materials, is more. The leakage trace index is a quantitative comparison of the leakage trace formed by the dripping of water contaminated liquid onto the surface of insulating materials.
This quantitative index has been applied to product design.
III Verification of insulation coordination
At present, the preferred method to verify insulation coordination is to use impulse dielectric test. Different rated impulse voltage values can be selected for different equipment.
1. verify the insulation coordination of the equipment with the rated impulse voltage test
1.2/50 of rated impulse voltage μ S waveform.
This waveform is used to simulate transient overvoltage and atmospheric overvoltage, as well as the overvoltage generated by switching on and off of low-voltage equipment. The output impedance of impulse test power supply pulse waveform generator shall generally be greater than 500 Ω. The determination of rated impulse voltage value shall be determined according to the equipment use occasion, overvoltage category and long-term service voltage of the equipment, and shall be corrected according to the corresponding altitude. At present, some test conditions for low-voltage switchgear assemblies. If the humidity and temperature are not clearly specified, they should also be within the scope of application of the complete set of switchgear standards. If the service environment of the equipment exceeds the scope of application of the complete set of switchgear, it must be considered for correction. The correction relationship between air pressure and temperature is as follows:
K=P/101.3 × 293( Δ T＋293)
K - correction parameter of air pressure and temperature
Δ T - temperature difference K between actual (Laboratory) temperature and t=20 ℃
P - actual air pressure kPa
2. dielectric test instead of impulse voltage
For low-voltage switchgear, AC or DC test can be used to replace impulse voltage test, but such test method is more severe than impulse voltage test, and shall be approved by the manufacturer.
AC test, under AC conditions, the duration is 3 cycles.
For DC test, apply voltage to each phase (positive and negative) for three times, each time lasting for 10ms.
IV General procedure for insulation coordination.
1. determination of typical overvoltage.
2. coordinate with determination of withstand voltage.
3. determination of rated insulation level.
Scanning code
